It is often necessary to screen particulate material in order to sort it in accordance with particle size. For example when mixing concrete or when building roads, it is necessary to screen gravel in order to sort into piles of different mean gravel size. Conventional screening apparatuses use screens and rotating or vibrating beds to select given particle size output. All particles which are of size equal or smaller to the selected size fall through the screen while the particles of larger size are held on the screen for later removal. One notable disadvantage with the rotating or vibrating bed type screening apparatuses is that they have a tendency to become clogged.